1. Field of the Invention
The present invention generally relates to downhole tools for use in a hydrocarbon wellbore. More particularly, this invention relates to an apparatus useful in performing a wellbore treatment operation. More particularly still, this invention relates to a pack-off system for effectively isolating an area of interest within a wellbore so that a treatment fluid may be pumped into the pack-off system and into the area of interest, and a method for using the same.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. When the well is drilled to a first designated depth, a first string of casing is run into the wellbore. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. Typically, the well is drilled to a second designated depth after the first string of casing is set in the wellbore. A second string of casing, or liner, is run into the wellbore to the second designated depth. This process may be repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing having an ever-decreasing diameter.
After a well has been drilled, it is desirable to provide a flow path for hydrocarbons from the surrounding formation into the newly formed wellbore. Therefore, after all of the casing has been set, perforations are shot through a wall of the liner string at a depth which equates to the anticipated depth of hydrocarbons. Alternatively, a liner having pre-formed slots may be run into the hole as casing. Alternatively still, a lower portion of the wellbore may remain uncased so that the formation and fluids residing therein remain exposed to the wellbore.
In many instances, either before or after production has begun, it is desirable to inject a treating fluid into the surrounding formation at particular depths. Such a depth is sometimes referred to as an area of interest in a formation. Often perforations formed within a wellbore to recover hydrocarbons from the surrounding formation become obstructed partially or completely. In such a situation, treating fluids under pressure may be introduced into the wellbore so that treating fluid is forced into the perforations and into the surrounding formation. The treating fluid removes the obstructions from the perforations, unclogging the perforations and repairing the wellbore so that hydrocarbons may again be recovered through the formation. Various treating fluids are known, such as acids, polymers, and fracturing fluids. Methods of injection of treating fluid into the wellbore are known as well treatment operations.
To perform a well treatment operation, the treating fluid must be introduced into the wellbore at a pressure sufficient to overcome the pressure created by the hydrocarbons exiting from the perforations in the wellbore during the recovery operation. Treatment fluids are expensive, and decreasing the area through which the treating fluid must flow decreases the amount of pressure necessary to overcome the pressure created by the exiting hydrocarbons. Therefore, it is often desirable to “straddle” the area of interest within the wellbore to decrease the volume of the treating fluid necessary to perform the well treatment operation. This is typically done by “packing off” the wellbore above and below the area of interest. To accomplish this, a first packing element is set above the area of interest, and a second packing element is set below the area of interest. Treating fluids can then be injected under pressure into the formation between the two set packing elements.
A variety of pack-off systems are available which include two selectively-settable and spaced-apart packing elements. Several such prior art systems use a piston or pistons movable in response to hydraulic pressure in order to actuate the setting apparatus for the packing elements. A different type of straddle pack-off system is disclosed in U.S. Pat. No. 6,253,856 B1, which is incorporated in its entirety herein by reference. This pack-off system does not require mechanical pulling and/or pushing in order to actuate the packing elements; rather, the packing elements are set through a combination of hydraulic and mechanical pressure. A specialized collar for use with the pack-off system of U.S. Pat. No. 6,253,856 is disclosed in the co-pending application “Fracturing Port Collar for Wellbore Pack-Off System, and Method for Using the Same,” U.S. Ser. No. 10/073,685, which is also incorporated herein by reference. The packing elements of the current invention may be used in combination with the any of the above pack-off systems, as well as in any other prior art pack-off systems which apply compressive force to the packing elements to expand the elements radially.
The packing elements of the prior art pack-off systems are expanded radially to sealably engage the inner diameter of the casing. These packing elements completely obstruct the flow of fluid through the annular space between the pack-off system and the casing. To accomplish the complete obstruction of fluid flow through the annular space between the pack-off system and the casing, the packing elements of the prior art are either inflatable or elastomeric. The inflatable packing elements are radially expanded hydraulically downhole by introducing fluid into the packing elements themselves. Elastomeric packing elements, which are made of an elastomeric material such as rubber, are radially expanded downhole by mechanical and/or hydraulic force. The mechanical force is essentially axial force which is exerted upward and downward on each packing element, thereby compressing each elastomeric packing element and forcing the packing element radially outward. Each type of packing element may be actuated by mechanical or hydraulic force or a combination of mechanical and hydraulic force.
Often, multiple areas of interest must be treated within a wellbore. To move the pack-off system to a second area of interest within the wellbore, the packing elements must experience a decrease in diameter by the release of compressive forces upon the packing elements. The pack-off system is then moved to another location within the wellbore so that the packing elements are again located above and below the second area of interest. Next, the packing elements must again be expanded radially to sealably engage the inner diameter of the casing above and below the second area of interest. This process is repeated to treat subsequent areas of interest within a wellbore.
While the packing elements of the prior art pack-off systems provide the advantage of completely sealing off fluid flow through the annular space between the pack-off system and the casing, these packing elements do possess certain disadvantages. Both elastomeric and inflatable packing elements lack durability. Specifically, upon treatment of multiple areas of interest, elastomeric and inflatable packing elements often lose strength and durability due to the stress exerted upon the packing elements during every compression and subsequent decompression required to treat each area of interest. Loss of strength and durability in the packing elements decreases the ability of the packing elements to sealably engage the casing to isolate subsequent areas of interest to perform the packing operation. Accordingly, the packing elements must often be replaced in order to treat more areas of interest. The pack-off system must be removed from the wellbore to replace the defective packing elements with new packing elements when the effectiveness of the packing elements is decreased. Then, the pack-off system must again be run into the wellbore. Every separate run-in of the pack-off system necessary to maintain the packing elements in good repair is extremely expensive due to labor and material costs.
Therefore, a need exists for durable packing elements for use in a pack-off system which are capable of treating multiple areas of interest within the wellbore with only one run-in of the pack-off system. There is a need for packing elements for use in a pack-off system which may be moved within the wellbore to treat multiple areas of interest while the packing elements are set. Decreasing the amount of times the packing elements must be compressed and decompressed allows treatment of multiple areas of interest within the wellbore upon one run-in of the pack-off system, decreasing the cost of the treatment operation.